Use of novel cytokine receptors as biomarkers and therapeutic targets in human cancer

ABSTRACT

Nucleic acids encoding erythropoietin isoforms are described herein, as well as the encoded isoforms, methods of detecting the same, and methods of screening for and treating cancer.

FIELD OF THE INVENTION

The present invention concerns nucleic acids encoding erythropoietinreceptor isoforms, proteins encoded by such nucleic acids, antibodiesthat bind to such proteins, and methods of using the same.

BACKGROUND OF THE INVENTION

Erythropoietin (Epo) is the principal hematopoietic growth factor thatpromotes the viability, differentiation and proliferation of mammalianerythroid progenitor cells (S. Krantz, Blood 77, 419-34 (1991)). Thebiologic effects of Epo are mediated via its interaction with itsspecific transmembrane receptor, EpoR (H., Youssoufian, Blood 81,2223-36 (1993)). The EpoR lacks intrinsic tyrosine kinase activity andupon ligand binding activates a receptor-associated tyrosine kinase Jak2which is critical for anti-apoptosis and mitogenic signaling via theEpoR (O. Miura et al., Blood 84, 1501-7 (1994); B. Witthuhn et al., Cell74, 227-36 (1993); J. Ihle, Nature 337, 591-4 (1995); H. Zhuang et al.,J. Biol Chem. 270, 14500-4 (1995)). Activated Jak2 then phosphorylates anumber of cytoplasmic proteins as well as the EpoR itself. Expression ofEpo receptors has been reported on several non-hematopoietic cell typesincluding vascular endothelial cells, placental tissue, neuronal cells,kidney and cardiomyocytes (A. Anagnostou et al., Proc. Natl. Acad. Sci.USA 91, 3974-8 (1994); S. Masuda et al., J. Biol. Chem. 268, 112-8-16(1993); S. Sawyer et al., Blood74, 103-9 (1989); M. Wald et al., J.Cell. Physiol. 167, 461-8 (1996)).

Recombinant human Epo (r-HuEpo) has been widely used in many differenttypes of cancers for the treatment or prevention of chemo-radiotherapyinduced anemia (A. Moliterno and J. Spivak, Hematol. Oncol. Clin. NorthAm. 10, 345-63 (1996)). For instance, in patients with breast cancer,r-HuEpo has been investigated in clinical trials for its potentialbeneficial effects in the prevention or treatment of chemotherapy orradiation therapy-related anemia (L. Del Mastro et al., J. Clin. Oncol.15, 2715-21 (1997); H. Ludwig et al., Ann. Oncol. 4, 161-7 (1993); P.Sweeney et al., Br. J. Cancer 77, 1996-2002 (1998); S. Vijayakumar etal., Int. J. Radiat. Oncol. Biol. Phys 26, 721-9 (1993)), formobilization of peripheral blood progenitor cells (C. Waller et al.,Bone Marrow Transplant 24, 19-24 (1999)), to increase the rate ofhematopoietic recovery following high dose chemotherapy (P. BenedettiPanici et al., Br. J. Cancer 75, 1205-12 (1997); S. Filip et al.,Neoplasma 46, 166-72 (1999)) as well as use in ex vivo expansionstrategies of stem cells (C. Bachier et al., Exp Hematol. 27, 615-23(1999); L. Pierelli et al., Exp. Hematol. 27, 416-24 (1999); P. Stiff etal., Blood 95, 2169-74 (2000); W. Vogel et al., Blood 86, 1362-7(1996)). Similarly, r-HuEpo has been investigated in several clinicaltrials of squamous cell cancers of head-neck (F. Dunphy et al., Cancer86, 1362-7 (1999); M. Henke et al., radiother Oncol 50, 185-90 (1999);G. Mantovani et al., Oncol. Rep. 6, 421-6 (1999)) and uterine cervix (K.Dusenbery et al., Int. J. Radiat. Oncol. Biol. Phys. 29, 1079-84(1994)).

In view of the foregoing, it would be extremely desirable to understandthe association of Epo with tumor growth and how EpoR may be involved incancer pathophysiology and progression.

SUMMARY OF THE INVENTION

A first aspect of the present invention is an isolated nucleic acidencoding erythropoietin isoform 1, erythropoietin isoform 2,erythropoietin isoform 3, erythropoietin isoform 4, or erythropoietinisoform 5, or a nucleic acid that encodes the opposite or complementarystrand of a nucleic acid as set forth above (e.g., a DNA encoding anRNA).

A second aspect of the present invention is a protein encoded by anucleic acid as described above (e.g., an isolated and/or purifiedprotein).

A third aspect of the present invention is an antibody that selectivelyor specifically binds to a protein as described above.

A further aspect of the present invention is an oligonucleotide probethat selectively or specifically binds to a nucleic acid as describedabove.

A further aspect of the present invention is a method of screening asubject for cancer, comprising: detecting the presence or absence of anucleic acid encoding an isoform as described above in the subject, thepresence of such a nucleic acid indicating the subject is afflicted withor at risk of developing cancer.

A further aspect of the present invention is a method of screening asubject for cancer, comprising detecting the presence or absence of aprotein or isoform as described above in the subject, the presence ofsuch a protein indicating the subject is afflicted with or at risk ofdeveloping cancer.

Particular cancers which may be screened by the methods described hereininclude, but are not limited to, breast, cervix, ovarian, prostate,colon and lung cancer.

The foregoing and other objects and aspects of the present invention areexplained in detail in the drawings herein and the specification setforth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. The organization of the EpoR gene (GenBank accession numberS45332, SEQ ID NO: 1). The splicing that results in the mature mRNA forthe wild-type receptor (SEQ ID NO: 3), and five alternatively splicedisoforms (1-5) described herein are depicted schematically. Thetranslated regions of the gene are indicated in black, whereasuntranslated regions are indicated in white. Novel amino acidtranslations that result from alternative splicing of the EpoR genetranscript are indicated in grey.

FIG. 2. Changes in the open reading frames (ORFs) of mature mRNAsequences from the full-length wild-type receptor in the isoforms ofEpoR described herein. Isoform 1 (SEQ ID NO: 4): Additional nucleotidesfrom intron 6 (nucleotides 5949-6062, SEQ ID NO: 1) are spliced betweenexons 6 and 7. Isoform 2 (SEQ ID NO: 6): Splicing at the 5′ end of exon8 occurs 19 nucleotides upstream (nucleotide 7498) from that seen in thefull-length wild-type message (nucleotide 7517). Isoform 3 (SEQ ID NO:8): Intron 7 is not spliced out of the final message. Isoform 4 (SEQ IDNO: 10): Intron 5 is not spliced out of the final message. Isoform 5(SEQ ID NO: 12): exon 6 is skipped, with exon 5 spliced directly to exon7. Putative C-terminal amino acid sequence changes from wild-type EpoRare depicted in bold.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is explained in greater detail below. Thisdescription is not intended to be a detailed catalog of all thedifferent ways in which the invention may be implemented or all thefeatures that may be added to the instant invention. For example,features illustrated with respect to one embodiment may be incorporatedinto other embodiments, and features illustrated with respect to aparticular embodiment may be deleted from that embodiment. In addition,numerous variations and additions to the various embodiments suggestedherein will be apparent to those skilled in the art in light of theinstant disclosure which do not depart from the instant invention.Hence, the following specification is intended to illustrate someparticular embodiments of the invention, and not to exhaustively specifyall permutations, combinations and variations thereof.

Nucleic acid as used herein refers to any type of nucleic acid,including naturally occurring and synthetic nucleic acids and includingboth DNA and RNA.

Subjects with which the present invention may be carried out aregenerally mammalian subjects, including both human subjects andnon-human subjects (e.g. dog, cat, horse, rabbit, rat) for veterinary orresearch purposes.

Any type of antibody may be used in the present invention. The term“antibodies” as used herein refers to all types of immunoglobulins,including TgG, IgM, IgA, IgD, and IgE. Of these, IgM and IgG areparticularly preferred. The antibodies may be monoclonal or polyclonal(with monoclonal antibodies preferred) and may be of any species oforigin, including (for example) mouse, rat, rabbit, horse, or human.See, e.g., M. Walker et al., Molec. Immunol. 26, 403-11 (1989). Antibodyfragments that retain specific binding to the protein or epitope boundby the antibody are included within the scope of the term “antibody” andinclude, for example, Fab, F(ab′)2, and Fc fragments, and thecorresponding fragments obtained from antibodies other than IgG. Suchfragments can be produced by known techniques. The antibodies may bechimeric or humanized, particularly when they are used for therapeuticpurposes.

Applicants specifically intend that all United States patent referencescited herein be incorporated herein by reference in their entirety.

1. Nucleic acids.

As noted above, a first aspect of the present invention is a nucleicacid encoding an erythropoieitin receptor isoform as described herein.In certain embodiments the nucleic acid may be an RNA such as an mRNA,or may be a DNA.

In one embodiment, the nucleic acid encodes erythropoietin receptorisoform 1 and has the sequence given herein as SEQ ID NO: 4.

In another embodiment, the nucleic acid encodes erythropoietin receptorisoform 2 and has the sequence given herein as SEQ ID NO: 6.

In another embodiment, the nucleic acid encodes erythropoietin receptorisoform 3 and has the sequence given herein as SEQ ID NO: 8.

In another embodiment, the nucleic acid encodes erythropoietin receptorisoform 4 and has the sequence given herein as SEQ ID NO: 10.

In another embodiment, the nucleic acid encodes erythropoietin receptorisoform 5 and has the sequence given herein as SEQ ID NO: 12.

In another embodiment, the nucleic acid that encodes the opposite strandof a nucleic acid as set forth above (e.g., is a DNA encoding an RNA).

Nucleic acids as described above may be natural or synthetic, and can beproduced in accordance with techniques known in the art or variationsthereof which will be apparent in light of the disclosure herein.

Nucleic acids as described above may be coupled to appropriateregulatory elements such as a promoter to produce a recombinant nucleicacid construct, which construct may be inserted into a host cell inwhich the promoter is operable so that the encoded protein is expressedby the host cell. Recombinant techniques and the production of proteinsin recombinant cells may be carried out in accordance with knowntechniques.

2. Antibodies.

Polyclonal antibodies used to carry out the present invention may beproduced by immunizing a suitable animal (e.g., rabbit, goat, etc.) withthe antigen to which the monoclonal antibody binds, collecting immuneserum from the animal, and separating the polyclonal antibodies from theimmune serum, in accordance with known procedures. Depending on the hostspecies, various adjuvants may be used to increase immunologicalresponse. Such adjuvants include, but are not limited to, Freund's,mineral gels such as aluminum hydroxide, and surface active substancessuch as lysolecithin, pluronic polyols, polyanions, peptides, oilemulsions, keyhole limpet hemocyanin, and dinitrophenol. Among adjuvantsused in humans, BCG (bacilli Calmette-Guerin) and Corynebacterium parvumare especially preferable.

Monoclonal antibodies of the present invention may be prepared using anytechnique which provides for the production of antibody molecules bycontinuous cell lines in culture. These include, but are not limited to,the hybridoma technique, the human B-cell bybridoma technique, and theEBV-hybridoma technique (Kohler, G. et al. (1975) Nature 256:495-497;Kozbor, D. et al. (1985) J. Immunol. Methods 81:31-42; Cote, R. J. etal. (1983) Proc. Natl. Acad. Sci. 80:2026-2030; Cole, S. P. et al.(1984) Mol. Cell Biol. 62:109-120). Briefly, the procedure is asfollows: an animal is immunized with antigen or immunogenic fragments orconjugates thereof. For example, haptenic oligopeptides of antigen canbe conjugated to a carrier protein to be used as an immunogen. Lymphoidcells (e.g. splenic lymphocytes) are then obtained from the immunizedanimal and fused with immortalizing cells (e.g. myeloma orheteromyeloma) to produce hybrid cells. The hybrid cells are screened toidentify those which produce the desired antibody.

Human hybridomas which secrete human antibody can be produced by theKohler and Milstein technique. Although human antibodies are especiallypreferred for treatment of human, in general, the generation of stablehuman-human hybridomas for long-term production of human monoclonalantibody can be difficult. Hybridoma production in rodents, especiallymouse, is a very well established procedure and thus, stable murinehybridomas provide an unlimited source of antibody of selectcharacteristics. As an alternative to human antibodies, the mouseantibodies can be converted to chimeric murine/human antibodies bygenetic engineering techniques. See V. T. Oi et al., Bio Techniques4(4):214-221 (1986); L. K. Sun et al., Hybridoma 5 (1986).

In addition, techniques developed for the production of “chimericantibodies”, the splicing of mouse antibody genes to human antibodygenes to obtain a molecule with appropriate antigen specificity andbiological activity can be used (S. L. Morrison, et al. Proc. Natl.Acad. Sci. 81, 6851-6855 (1984); M. S. Neuberger et al., Nature312:604-608 (1984); S. Takeda, S. et al., Nature 314:452-454 (1985)).Alternatively, techniques described for the production of single chainantibodies may be adapted, using methods known in the art, to produceisoform-specific single chain antibodies. Antibodies with relatedspecificity, but of distinct idiotypic composition, may be generated bychain shuffling from random combinatorial immunoglobin libraries (D. R.Burton, Proc. Natl. Acad. Sci. 88,11120-3 (1991)).

Antibodies may also be produced by inducing in in vivo production in thelymphocyte population or by screening immunoglobulin libraries or panelsof highly specific binding reagents as disclosed in the literature (R.Orlandi et al., Proc. Natl. Acad. Sci. 86, 3833-3837 (1989)); G. Winteret al., Nature 349, 293-299 (1991)).

Antibodies that selectively bind to a particular erythropoietin receptorisoform as described herein (i.e., that selectively bind to one ofisoforms 1-5 but do not bind to the other of isoforms 1-5) can beidentified in accordance with known techniques, such as their ability tocompete with labeled antibody to in binding to that isoform in acompetitive binding assay.

If desired, antibodies specific for a particular isoform can be used toproduce anti-idiotypic (paratope-specific) antibodies. See e.g.,McNamara et al., Science 220,1325-26 (1984), R. C. Kennedy, et al.,Science 232,220 (1986).

3. Immunoassay Techniques.

Those skilled in the art will be familiar with numerous specificimmunoassay formats and variations thereof which may be useful forcarrying out the method disclosed herein. See generally E. Maggio,Enzyme-Immunoassay, (1980)(CRC Press, Inc., Boca Raton, Fla.); see alsoU.S. Pat. No. 4,727,022 to Skold et al. titled “Methods for ModulatingLigand-Receptor Interactions and their Application,” U.S. Pat. No.4,659,678 to Forrest et al. titled “Immunoassay of Antigens,” U.S. Pat.No. 4,376,110 to David et al., titled “Immunometric Assays UsingMonoclonal Antibodies,” U.S. Pat. No. 4,275,149 to Litman et al., titled“Macromolecular Environment Control in Specific Receptor Assays,” U.S.Pat. No. 4,233,402 to Maggio et al., titled “Reagents and MethodEmploying Channeling,” and U.S. Pat. No. 4,230,767 to Boguslaski et al.,titled “Heterogenous Specific Binding Assay Employing a Coenzyme asLabel.”

Antibodies as described herein may be coupled or conjugated to a solidsupport suitable for a diagnostic assay (e.g., beads, plates, slides orwells formed from materials such as latex or polystyrene) in accordancewith known techniques, such as precipitation. Antibodies as describedherein may likewise be coupled or conjugated to detectable groups suchas radiolabels (e.g., ³⁵S, ¹²⁵I, ¹³¹I), enzyme labels (e.g., horseradishperoxidase, alkaline phosphatase), fluorescent labels (e.g.,fluorescein), chemiluminescent labels (e.g., acridinium groups,metalloporphyrins such as phthalocyanine dyes, luminol, etc.), metalatoms (e.g., technetium-99m), etc., in accordance with known techniques.See, e.g., U.S. Pat. No. 4,472,509 to Gansow (metal chelates tomonoclonal antibodies); U.S. Pat. No. 5,061,641 to Schochat et al.; andU.S. Pat. No. 4,861,869 to Nicoleotti et al. (radiolabelling proteins).

Immunoassays, or other types of assays to detect and/or quantitate thelevel of the isoform in samples as described below, may be used inscreening assays to detect pathologic states associated with aberrantlevels of isoform expression (e.g., tumors, inflammatory states),diagnostic studies, prognostic studies, or to monitor the progression ordiminution of isoform expression in correlation with disease state.

Samples that may be collected for use in carrying out the immunoassaymay be tissue samples from the organ or tissue of interest within thesubject, such tissue generally of most interest being those types oftissues/cells that express differing amounts of isoform in pathologicstates as compared to non-pathologic states, or biological fluids suchas blood (including blood fractions such as blood plasma or bloodserum), urine, cerebrospinal fluid, etc). Examples may includeoverexpression or aberrant expression of the isoform in various types ofmalignancies (e.g ovarian cancer, endometrial cancer, pancreatic cancer,breast cancer, urinary bladder cancer, lung cancer, etc.), as well asoverexpression or aberrant expression in other pathologic states.

A biological sample may be a cell sample, with an intervening culturingstep being performed between the time the cell sample is collected fromthe subject and the immunoassay is carried out on the biological sample.

For immunohistological techniques, a tissue sample is collected from thesubject, and the presence or absence of binding of an antibody of theinvention is detected. The presence of binding of the antibody in anabnormal pattern or a pattern indicative of a tumor or cancer indicatesthe presence of a tumor or cancer in the subject from which the tissuesample is collected. The presence of the antigen in a metastatic tumordeposit can also be used to determine a likely source of the primarytumor. Any suitable immunohistology format may be used. The tissuesample may include patient biopsies, resections or cells for cytologicstudy. A similar technique to immunohistology is the use of similartechniques to detect and/or phenotype cells in body fluids or othersuspensions as is used for flow cytometric examination.

For in vivo diagnostic purposes the antibody according to the inventionis coupled to or provided with a suitable externally detectable label,such as e.g. a radiolabel as described above or a metal atom (e.g.,technetium-99m), and administered to a subject (e.g., by intraveneous orintraarterial injection), in an amount sufficient to produce anexternally detectable signal, whereupon the possible localizedaccumulation of antibody in the body is determined, with a localizedaccumulation of the antibody (in a region other than that which wouldordinarily be expected for normal subjects or subjects free of disease)indicating the present of a tumor in that subject.

4. Nucleic Acid Assay Techniques.

Detection of mRNAs specific to EpoR isoforms 1, 2, 3, 4, and 5 may becarried out by any suitable technique, including but not limited tousing reverse transcriptase-polymerase chain reaction (RT-PCR)amplification with isoform-specific primers and Southern blot analysisof the resulting RT-PCR amplicons. For example, PolyA⁺ RNA may beisolated by any technique known by those skilled in the art frompatients patient cells and/or cancer cells, including but not limited tobreast, colon, lung, ovary, and prostate cells or cancer cells. Methodsfor RT-PCR amplification of the isolated RNA are known in the art andmay be carried out using EpoR isoform-specific primer pairs, preferablyas described below.

Oligonucleotide probes (or primers) that specifically bind to a nucleicacid encoding an isoform as described above (including the oppositestrands thereof), and pairs of probes (where at least one member of thepair is specific for a nucleic acid encoding one particular isoform),are also an aspect of the present invention. In general, such probes arefrom 8 or 10 nucleic acids in length up to 40 or 50 nucleic acids inlength, or more. By “specifically bind” is meant that a probe binds to anucleic acid (or complement thereof) that encodes one isoform asdescribed herein, but does not bind to a nucleic acid (or complementthereof) that encodes another isoform as described herein. Probes mayoptionally be labeled with a detectable group such as a radioisotope,enzyme, or member of a binding pair in some assay formats.

Where a pair of probes or primers is used for amplification, it will beappreciated that only one member of the pair need be isoform-specific,and that the other member of the pair may be one which will bind tonucleic acids encoding more than one of the isoforms described herein,so long as the primer pair specifically amplifies only nucleic acidencoding one of the isoforms described herein. Examples of sucholigonucleotide probes, and pairs thereof, are as follows: Primer pairspecific for intron 6 insert (isoform 1) (SEQ ID NO: 14) 28AS-: 5′ TCAAGC GGC TGC TTC CTT CCA A 3′ (SEQ ID NO: 15) ER4-5: 5′ GCA GGG AGC GTACAG AGG GTG GAG 3′ Primer pair specific for intron 7 insert (isoform 2)(SEQ ID NO: 16) 33AS: 5′ GAA GAA ATA GCA CCA ACC TGG AAG 3′ (SEQ ID NO:17) 31S: 5′ CTG ACG CCT AGC GAC CTG GAC C 3′ Primer pair specific forintron 7 unspliced (isoform 3) (SEQ ID NO: 18) 31AS: 5′ GCA GTT TGG CTGCAA GAA GCA 3′ (SEQ ID NO: 17) 31S: 5′ CTG ACG CCT AGC GAC CTG GAC C 3′Primer pair specific for intron 5 unspliced (isoform 4) (SEQ ID NO: 19)26S: 5′ GGA GCC AGG GCG AAT CAC GG 3′ (SEQ ID NO: 20) 32S: 5′ GCC TTCAAA CTC GCT CTC TG 3′ Primer pair specific for exon 6 skipped (isoform5) (SEQ ID NO: 21) 34AS 5′ GCT TCA GAG CCC GCT AGG CGT 3′ (SEQ ID NO:15) ER4-5 5′ GCA GGG AGC GTA CAG AGG GTG GAG 3′Note that, in the foregoing pairs, the primers of SEQ ID NO. 14, 16, 18,19 and 21 are specific for the identified isoform, and the primers ofSEQ ID NO. 15, 17 and 20 are not specific. In each pair, only one primerneed be specific to provide an isoform-specific primer pair.

Blotting techniques are well known in the art. See, e.g., Sambrook etal., Molecular Clooning: a Laboratory Manual 3rd Ed. (Cold SpringHarbor, NY.); Ausubel et al. Current Protocols in Molecular Biology(Green Publishing Associates, Inc. and John Wiley & Sons, Inc., NewYork). The nucleic acids resulting from RT-PCR amplification may beseparated by gel electrophoresis and immobilized on a suitable matrix,e.g. a filter of nitrocellulose. The presence of target sequences amongthe amplification products may be shown by incubation of the blottedamplicons with a probe (usually labeled) under conditions that promotedenaturation and rehybridization. Because the probe is designed to basepair with target sequences, the probe will bind under renaturingconditions. Unbound probe is then removed, and detection of targetsequences may be accomplished via known techniques to detect the labeledprobe.

The present invention and the various methods and compounds therein areexplained in greater detail in the following non-limiting examples.

EXAMPLE 1

The Erythropoietin Receptor (EpoR) gene. The human EpoR gene has beencloned and sequenced as previously described (Noguchi et al. (1991)Blood 78:2548-2556). The gene spans 8.6 kilobases, and comprises of 8exons with 7 intervening introns, the latter of which range in size from81 bp to 2.1 kb. The organization of the EpoR gene is outlined inFIG. 1. The full-length wild-type form of EpoR comprises of 508 aminoacids (SEQ ID NO: 2) in three domains: extracellular, transmembrane(TM), and cytoplasmic. Exons 1-5 encode for the extracellular domain ofEpoR, exon VI encodes for the transmembrane domain, while exons VII andVIII encode for the cytoplasmic domain of the receptor. In theexamination of EpoR expression in tumor vasculature, analysis by RT-PCRindicated a high level of EpoR mRNA expression in breast cancer cells,as well as squamous cell cancers of head-neck and uterine cervix wasobserved.

EXAMPLE 2

Novel Isoforms of EpoR mRNA Transcripts. The resulting RT-PCRamplification products derived from human cervix, breast, prostate, andovarian cancer cell lines were sequenced and analyzed. The results ofthis study revealed five alternatively spliced EpoR mRNA transcriptsthat differ from the mature, full-length wild-type EpoR mRNA. Usingisoform-specific PCR primers, transcripts corresponding to each isoformwere detected in breast, colon, lung, ovarian and prostate cancer. Theorganization of these five isoforms is outlined in FIG. 1. Thealternative forms of EpoR predicted to be coded for from thesealternatively spliced mRNAs fall into two categories isoforms 1, 2, and3 are described as truncated (EpoR-T), and possess the extracellular andtransmembrane domains of the wild-type receptor, while lacking portionsof the cytoplasmic domain. Isoforms 4 and 5 are described as soluble(EpoR-S), and only possess the extracellular domain of the wild-typereceptor intact. The changes in the putative C-terminal amino acidsequence encoded by these mRNAs are outlined in FIG. 2.

EXAMPLE 3

EpoR Isoform 1. The mRNA that codes for Isoform 1 contains an additional114 nucleotides from intron 6 (nucleotides 5949-6062, SEQ ID NO: 1)spliced between exons 6 and 7. The resulting mRNA will code for an EpoRpeptide 285 amino acids in length (SEQ ID NO: 5) with a severetruncation in the cytoplasmic region. At the C-terminal, 9 novel aminoacids (M V R E G S R R R STOP) inserted at position 277 of thefull-length EpoR peptide sequence.

EXAMPLE 4

EpoR Isoform 2. The mRNA that codes for Isoform 2 is the result of analternative splicing event between the 3′ end of exon 7 and 5′ end ofexon 8, in which an additional 19 nucleotides (nucleotides 7498-7516,SEQ ID NO: 1) are added to the 5′ end of exon 8. The mRNA from thissplicing event codes for an EpoR peptide 317 amino acids in length (SEQID NO: 7) with a severe truncation in the cytoplasmic domain, in which12 novel amino acids (V G A I S S A V A V P E STOP) are inserted atposition 306 of the EpoR peptide sequence. As with Isoform 1, Isoform 2also possesses a truncation of the cytoplasmic domain of the full-lengthpeptide sequence of EpoR.

EXAMPLE 5

EpoR Isoform 3. The translation of isoform 3 results from a processedEpoR mRNA in which sequences from intron 7 (nucleotides 7422-7516, SEQID NO: 1) are not spliced out of the final message. The resultingtranslation is a 328 amino acid peptide (SEQ ID NO: 9), with 23 novelamino acids introduced to the C-terminus (V G G L V V P S V P G L P C FL Q P N C R P L STOP) at position 306 of the EpoR peptide sequence. Aswith Isoforms 1 and 2, Isoform 3 possesses a truncation of thecytoplasmic domain of the full-length peptide sequence of EpoR. Thesequence of the ORF of the mRNA message (SEQ ID NO: 8) and the peptidesequence of EpoR Isoform 3 (SEQ ID NO: 9) is identical to thetranslation predicted from an mRNA described previously (Nakamura et al.(1992) Science 257:1138-1141).

EXAMPLE 6

EpoR Isoform 4. The processed EpoR mRNA that translates into Isoform 4contains sequences from intron 5 (nucleotides 5061-5144, SEQ ID NO: 1)are not spliced out of the final message. The resulting translation is a267 amino acid peptide (SEQ ID NO: 11) with 21 novel amino acids (G E AP G G G V G G A R A N H G A S P P P STOP) introduced to the C-terminusat position 247 of the full-length EpoR peptide sequence. This isoformof EpoR possesses neither the transmembrane nor cytoplasmic domains ofthe full-length receptor. The translation that codes for Isoform 4results in a soluble form of EpoR, containing the extracellular domainof the receptor only.

EXAMPLE 7

EpoR Isoform 5. Isoform 6 is a translation that results from thealternatively processed EpoR mRNA in which sequences from exon 6 areskipped, i.e. exons 5 and 7 are spliced together directly. Thetranslation of this message results in a 248 amino acid peptide (SEQ IDNO: 13), in which 2 novel amino acids (G L STOP) are introduced atposition 247 of the full-length peptide sequence of EpoR. As withIsoform 4, Isoform 5 of EpoR is a soluble form of the receptor thatcomprises of only the extracellular domain.

The foregoing is illustrative of the present invention, and is not to beconstrued as limiting thereof. The invention is defined by the followingclaims, with equivalents of the claims to be included therein.

1. An isolated nucleic acid selected from the group consisting of anucleic acid encoding erythropoietin receptor isoform 2 and having thesequence given herein as SEQ ID NO: 6; a nucleic acid encodingerythrropoietin receptor isoform 3 and having the sequence given hereinas SEQ ID NO: 8; a nucleic acid encoding erythropoietin receptor isoform4 and having the sequence given herein as SEQ ID NO: 10; a nucleic acidencoding erythropoietin receptor isoform 5 and having the sequence givenherein as SEQ ID NO: 12; and a nucleic acid that is the full lengthcomplement of SEQ ID NO:6.
 2. (canceled)
 3. The nucleic acid accordingto claim 1 encoding erythropoietin receptor isoform 2 and having thesequence given herein as SEQ ID NO:
 6. 4. The nucleic acid according toclaim 1 encoding erythropoietin receptor isoform 3 and having thesequence given herein as SEQ ID NO:
 8. 5. The nucleic acid according toclaim 1 encoding erythropoietin receptor isoform 4 and having thesequence given herein as SEQ ID NO:
 10. 6. The nucleic acid according toclaim 1 encoding erythropoietin receptor isoform 5 and having thesequence given herein as SEQ ID NO:
 12. 7. The nucleic acid according toclaim 1, wherein said nucleic acid is an RNA.
 8. A recombinant nucleicacid comprising a promoter operatively associated with a nucleic acidaccording to claim
 1. 9. An isolated host cell in culture containing arecombinant nucleic acid according to claim 8 and which expresses theencoded erythropoetin receptor isoform. 10-16. (canceled)
 17. A methodof screening a subject for cancer, comprising: detecting the presence orabsence of a nucleic acid according to claim 1 in said subject, thepresence of such a nucleic acid indicating said subject is afflictedwith or at risk of developing cancer.
 18. The method according to claim17, wherein said cancer is breast, cervix, colon, lung, ovarian orprostate cancer.
 19. The method according to claim 17, wherein saiddetecting step is carried out by collecting a biological sample fromsaid subject, and then detecting the presence or absence of said nucleicacid in said biological sample.
 20. A method of screening a subject forcancer, comprising: detecting the presence or absence of a proteinencoded by a nucleic acid according to claim 1 in said subject, thepresence of such a protein indicating said subject is afflicted with orat risk of developing cancer.
 21. The method according to claim 20,wherein said cancer is breast, cervix, colon, lung, ovarian or prostatecancer.
 22. The method according to claim 20, wherein said detectingstep is carried out by collecting a biological sample from said subject,and then detecting the presence or absence of said protein in saidbiological sample.
 23. The method according to claim 20, wherein saiddetecting step is carried out by immunoassay.
 24. The method accordingto claim 20, wherein said detecting step is carried out by detectingnucleic acid that encodes said protein.